1. Field of the Invention
The present invention relates in general to signal detection. In particular the present invention relates to signal detection using sphere decoding techniques.
2. Description of the Related Art
Recently in the area of communications systems, multiple input multiple output (MIMO) technology has gained a lot of attention in the research community. An important and interesting research area of MIMO systems, and also in connection with other systems, is the detection of the received signals.
Detection of received signals refers to determining which signals were sent based on received signal. Using a vector notation, where in the case of a MIMO system each vector component xi of a sent signal x represents a signal (symbol) sent from one MIMO antenna, the aim in signal decoding is to determine a sent signal x based on and channel knowledge and a received signal r. A symbol xi needs to be a valid symbol of the modulation scheme used in the transmission. In principle, the modulation scheme symbol appearing due to channel distortions to be nearest to the received symbol ri is determined to be the sent symbol xi. Calculation of distances to all possible symbols is an extremely complicated task, so in practice the nearest symbol within a certain search area is selected as the sent symbol xi. The difficulty is to find this modulation scheme symbol nearest to the received symbol ri, or candidates for this nearest symbol, in an efficient way.
Different algorithms have been proposed, discussed and tested for signal detection. One of these signal detection algorithms is called a Sphere Decoder, and it has been proposed by E. Viterbo and J. Boutros in, “A Universal Lattice Code Decoder for Fading Channels”, IEEE Transactions on Information Theory, Vol. 45, No. 5, July 1999, pp. 1639-1642. The Sphere Decoder is originally presented for decoding a coded signal, but it is applicable also in signal detection. A sphere decoder is a sub-optimal maximum likelihood method with the advantage of low complexity. In the sphere coding, the signal components xi are determined one by one by searching a nearest valid modulation scheme symbol for a received symbol ri within a search area.
The basic idea in a Sphere Decoder is to process vectors and matrices representing the received symbols and channel knowledge so that interference between the sent symbols x1, x2, . . . , xN caused by a channel is taken into account and at the same time it is possible to determine a first symbol xN independently of the other symbols. Using the first determined symbol xN it is possible to determine symbol xN−1 and so on, resulting in a vector x containing symbols xi. The first determined symbol is denoted here with the index N, because the calculations in a Sphere Decoder typically involve upper-triangular matrices.
When information is transmitted and distorted in a noisy channel, the data becomes fuzzy and any decision made in the receiver side may lead to errors and lost of information. Soft detection has the target of keeping some reliability information on a detected symbol and making a “hard” decision as late as possible in the receiver. The known sphere decoders are designed as a “hard” output detector, returning as the sent signal x the vector of constellation symbols with the shortest Euclidean distance to the received signal r. Furthermore, it is possible that there is available some a priori information relating to the sent signal. This a priori information could enhance the accuracy of determining the sent signal x.
In many communication systems there are defined a number of modulation schemes, which can be used. The modulation scheme in use may vary from user to user, depending for example on the transmission rate relating to each user. Current sphere detection methods are not able to decode signals relating to different modulation schemes simultaneously.
There is thus a need for more versatile signal detection methods. The aim of the embodiments of this invention is to provide signal detection using sphere decoding for various purposes.
It is appreciated that although problems relating to signal detection using sphere decoding have been discussed in connection with MIMO systems, they may be relevant also in other communications systems.